1. Field of the Invention
The present invention relates to a liquid crystal display device and its manufacturing method, particularly to an art to be effectively applied to an in-plane field type active-matrix liquid crystal display device.
2. Description of the Prior Art
An active-matrix liquid crystal display device using an active element such as thin film transistor (TFT) has been widely spread as a display terminal of OA equipment because it is thin and lightweighted and has a high image quality equal to that of a cathode-ray tube.
The display system of the active-matrix liquid crystal display device is roughly divided into the following types.
One of them is a type, in which a liquid crystal layer is enclosed between a pair of substrates with two transparent electrodes formed on the substrates, a driving voltage is applied to the transparent electrodes, thereby driving the liquid crystal layer by an electric field almost perpendicular to the surfaces of the substrates, and the light passing the transparent electrodes and entering the liquid crystal layer is modulated (hereafter referred to as a vertical field type). Every product spread at present uses this type.
However, an active-matrix liquid crystal display device using the vertical field type has the problems on practical use that a contrast of an image extremely varies when changing viewing angles and particularly, a gradation level is inverted depending on a viewing angle when displaying half tone images.
The other of them is a type, in which a liquid crystal layer is enclosed between a pair of substrates, a driving voltage is applied to two stripe-like or line-like electrodes formed on either or both of the substrates, thereby driving a liquid crystal layer by an electric field almost parallel with the surfaces of the liquid crystal layer, and the light entering the liquid crystal layer from the gap between the two electrodes is modulated (hereafter referred to as an in-plane field type).
An active-matrix liquid crystal display device using the in-plane field type can realize wide viewing-angle characteristics. However, any active-matrix liquid crystal display device using the in-plane field type is not practically used yet.
Features of an active-matrix liquid crystal display device using the in-plane field type are shown in the official gazettes of Japanese Patent Application No. 505247/1993, Japanese Patent Publication No. 21907/1988, and Japanese Patent Laid-Open No. 160878/1994.
A conventional active-matrix liquid crystal display device using the in-plane field type modulates incoming light to a liquid crystal layer by rotating homogeneously initial-orienting liquid crystal molecules with no twisting, where an initial orientation direction is at an inclination to a pixel electrode and a counter electrode arranged in parallel, to create a reorientation state of liquid crystal molecules with twisting, whose major-axes are rotated substantially parallel with the surfaces of the liquid crystal layer, and displays images by a driving voltage enough small for conventional video signal drivers and with a response speed enough high to display animation.
Furthermore, the conventional active-matrix liquid crystal display device using the in-plane field type has extremely wide viewing angle characteristics compared with the active-matrix liquid crystal display device using the vertical field type.
However, the active-matrix liquid crystal display device using the in-plane field type cited above has a problem that viewing angle characteristics equal to those of a self-light-emitting display device such as a cathode ray tube (CRT) cannot be achieved because a homogeneous color tone cannot be realized and the viewing angle range of isochromaticity narrows when tilting a viewing angle to a certain direction.
That is, when liquid crystal molecules are twisted by rotation and viewing angle is tilted to the major-axis direction of the molecules, the birefringence anisotropy of the liquid crystal molecules more easily changes compared with the case of tilting the viewing angle to other directions, so that gradation level is more easily inverted and color tone more easily changes in the major-axis direction than in other directions.
Particularly, when a white image is displayed in the normally black mode, the color tone of white shifts to blue in the major-axis direction of the liquid crystal molecules.
Moreover, though the birefringence anisotropy does little change in the minor-axis direction of the liquid crystal molecules perpendicular to the major-axis direction of them, the color tone of white shifts to yellow in the minor-axis direction because the optical path length increases as the viewing angle tilts to the minor-axis direction.
The present invention has been made to solve the above mentioned problems of the prior art and its object is to provide an art for realizing wide viewing angle characteristics equal to those of a CRT and improving the image quality for an active-matrix liquid crystal display device using the in-plane field type.
The above and other objects and novel features of the present invention will become more apparent by the description of the present specification and the accompanying drawings.
The outline of a typical invention out of the inventions disclosed in this application is briefly described below.
(1) An active-matrix liquid crystal display device comprises a pair of substrates, a liquid crystal layer held between the substrates, a plurality of video signal lines formed on a first substrate of the pair, a plurality of scanning signal lines formed on the first substrate of the pair and intersecting the video signal lines, and a plurality of picture elements formed in a matrix in the intersecting regions between the video signal lines and the scanning signal lines;
wherein each of the picture elements has at least an active element formed on the first substrate, at least a pixel electrode connected to the active element, and at least a counter electrode formed on either of the substrates to generate an electric field almost parallel with the surfaces of the liquid crystal layer between the counter electrode and the pixel electrode;
and wherein liquid crystal molecules of the liquid crystal layer have at least two kinds of driving (reorientation) directions for neighboring picture elements or in one picture element.
(2) For the means in the above Item (1), the liquid crystal molecules of the liquid crystal layer between the counter electrode and the pixel electrode have one initial orientation direction.
(3) For the means in the above Item (2), each of the picture elements has a plurality of pairs of pixel electrodes and counter electrodes; wherein each pair of a pixel electrode and a counter electrode have a pair of facing sides faced almost parallel each other and the plurality of pairs of the facing sides have a tilt angle to the initial orientation direction of the liquid crystal molecules.
(4) For the mean in the above Item (3), wherein the initial orientation direction of the liquid crystal molecules is almost vertical to the scanning signal lines or parallel with the video signal lines, and picture elements with tilt angles xcex8 and xe2x88x92xcex8 are alternately arranged into a matrix.
(5) For the means in the above Item (4), the angle xcex8 is kept in a range of 10xc2x0xe2x89xa6xcex8xe2x89xa6200.
(6) For the means in the above Item (2), each of the picture elements has a plurality of pairs of pixel electrodes and counter electrodes; wherein each pair of a pixel electrode and a counter electrode have a pair of linear facing sides faced each other; and one of the pair of linear facing sides has a tilt angle to the initial orientation direction while the other of the pair is parallel with the initial orientation direction.
(7) For the means in the above Item (6), wherein the initial orientation direction of liquid crystal molecules is almost vertical to the scanning signal lines or parallel with the video signal lines, and the tilt angles of the plurality of pairs of facing sides are equal to xcex8 and xe2x88x92xcex8.
(8) For the means in the above Item (7), the angle xcex8 is kept in a range of 10xc2x0xe2x89xa6xcex8xe2x89xa620xc2x0, and the numbers of the pairs of facing sides with tilt angles of xcex8 and xe2x88x92xcex8 in each of the picture elements are the same.
(9) For the means in the above Item (2), each of the picture elements has a plurality of pairs of pixel electrodes and counter electrodes; wherein each pair of a pixel electrode and a counter electrode have a pair of facing sides faced each other, and a first side of the pair is almost parallel with the initial orientation direction while a second side of the pair is formed by two parts, one part being extended almost parallel with the initial orientation direction and the other part being tilted from the initial orientation direction at a tilt angle and intersecting with the first side at near the edge of the first electrode; and wherein the plurality of pairs of facing sides have a plurality of the tilt angles in each picture element.
(10) For the means in the above Item (9), wherein the initial orientation direction of liquid crystal molecules is almost vertical to the scanning signal lines or almost parallel with the video signal lines, and the plurality of the tilt angles are equal to xcex8 and xe2x88x92xcex8.
(11) For the means in the above Item (10), the angle xcex8 is kept in a range of 30xc2x0xe2x89xa6xcex8xe2x89xa660xc2x0, and the numbers of the pairs of facing sides with the tilt angles of xcex8 and xe2x88x92xcex8 in each of the picture elements are the same.
(12) For the means in the above Item (2), each of the picture elements has a plurality of pairs of pixel electrodes and counter electrodes; wherein each pair of a pixel electrode and a counter electrode have a pair of facing sides faced almost parallel each other and are bent inside the image display region of each of the picture elements.
(13) For the means in the above Item (12), wherein the video signal lines or the scanning signal lines are bent to be almost parallel with the pair of facing sides.
(14) For the means in the above Item (12), there are two or more types of gap distances between pairs of pixel electrodes and counter electrodes in each of the picture elements.
(15) For the means in the above Item (1), the liquid crystal molecules of the liquid crystal layer between the counter electrode and the pixel electrode have two initial orientation directions in each of the picture elements.
(16) For the means in the above Item (15), the liquid crystal layer has a positive dielectric anisotropy, initial orientation angles xcfx86 LC1 and xcfx86 LC2 are 90xc2x0+xcex1 and 90xc2x0xe2x88x92xcex1, respectively, and angles xcfx86 P1 and xcfx86 P2 between the transmission axes of two polarizing plates and the direction (EDR) of the applied electric field are 90xc2x0 and 0xc2x0 respectively.
(17) For the means in the above Item (15), the liquid crystal layer has a negative dielectric anisotropy, initial orientation angles xcfx86 LC1 and xcfx86 LC2 are 0xc2x0+xcex1 and 180xc2x0xe2x88x92xcex1, respectively, and angles xcfx86 P1 and xcfx86 P2 between the transmission axes of two polarizing plates and the direction (EDR) of the applied electric field are 90xc2x0 and 0xc2x0, respectively.
(18) For the means in the above Item (16) or (17), the absolute value of xcex1 is 2.5xc2x0 or less.
(19) For the means in the above Item (15), initial orientation angles xcfx86 LC1 and xcfx86 LC2 are 45xc2x0 and 135xc2x0, respectively, and angles xcfx86 P1 and xcfx86 P2 between the transmission axes of two polarizing plates and the direction (EDR) of the applied electric field are 90xc2x0 and 0xc2x0 respectively.
(20) For the means in the above Item (15), the boundary between the two initial orientation directions of liquid crystal molecules is arranged over a pixel electrode or a counter electrode in each of the picture elements.
(21) For the means in the above Item (2) or (15), wherein an initial twist angle of the liquid crystal layer is within 5 degrees of 0xc2x0.
(22) For a manufacturing method of an active-matrix liquid crystal display device comprising a pair of substrates, a liquid crystal layer held between the substrates, a plurality of active elements formed in a matrix on a first substrate of the pair, a plurality of pixel electrodes connected to the active elements respectively, a plurality of counter electrodes formed on either of the substrates to generate an electric field almost parallel with the surfaces of the liquid crystal layer between the pixel electrodes and the counter electrodes, a pair of orientation films formed between the substrates and contacting the liquid crystal layer, and two polarizing plates formed on surfaces opposite to the surfaces of the substrates for holding the liquid crystal layer; two-directional rubbings are applied to the both orientation films in one picture element.
(23) For a manufacturing method of an active-matrix liquid crystal display device comprising at least a pair of substrates, a liquid crystal layer held between the substrates, a plurality of active elements formed in a matrix on a first substrate of the pair, a plurality of pixel electrodes connected to the active elements respectively, a plurality of counter electrodes formed on either of the substrates to generate an electric field almost parallel with the surfaces of the liquid crystal layer between the pixel electrodes and the counter electrodes, a pair of orientation films formed between the substrates and contacting the liquid crystal layer, and two polarizing plates formed on surfaces opposite to the surfaces of the substrates for holding the liquid crystal layer; a chiral agent is mixed in the liquid crystal laver and two-directional rubbings are applied only to either of the orientation films in one picture element.
(24) For a manufacturing method of an active-matrix liquid crystal display device comprising at least a pair of substrates, a liquid crystal layer held between the substrates, a plurality of active elements formed in a matrix on a first substrate of the pair, a plurality of pixel electrodes connected to the active elements respectively, a plurality of counter electrodes formed on either of the substrates to generate an electric field almost parallel with the substrate surfaces to the liquid crystal layer between the pixel electrodes and the counter electrodes, a pair of orientation films formed between the substrates and contacting the liquid crystal layer, and two polarizing plates formed on surfaces opposite to the surfaces of the substrates for holding the liquid crystal layer; two initial orientation directions of liquid crystal molecules are provided in one picture element by applying a laser beam having two predetermined polarized directions to different regions of the orientation films in the picture element.
According to the above means, shifts of color tones are offset each other and the dependency of color-tone on a viewing angle can greatly be reduced because the initial orientation angle xcfx86 LC is made different for neighboring picture elements or in one picture element so as to form two or more kinds of reorientation directions.
For example, in an in-plane field type device utilizing the normally black mode, in which a displayed image is dark when no voltage is applied and bright when a voltage is applied, and also utilizing the birefringence first minimum mode, the transmission axes of the two polarizing plates are perpendicularly intersected each other (cross Nicols), and the maximum transmittance, that is, a white image is obtained when the angle formed between each transmission axis and the major axis of liquid crystal molecules twisted by the electric field becomes almost equal to 45xc2x0.
When changing viewing directions from an upward direction, which is vertical to the substrate surface, to a tilted direction toward the substrate surface in the major-axis direction of liquid crystal molecules in the direction of about 45xc2x0 away from the transmission axis under the above twisted state, the birefringence anisotropy changes and the color tone of white shifts to blue in the major-axis direction.
In the minor-axis direction of liquid crystal molecules (at a direction of about xe2x88x9245xc2x0 away from the transmission axis, and perpendicular to the major-axis direction), the birefringence anisotropy does little change by tilting viewing angles from a vertical direction to an in-plane direction.
However, the color tone of white shifts to yellow in the minor-axis direction because the optical path length increases as the viewing angle tilts from a vertical to a in-plane direction in the minor-axis direction.
The important point is that, because blue and yellow colors are complimentary colors in chromaticity coordinates, white color can be created by mixing these two colors.
Therefore, by rotating liquid crystal molecules in two directions for each picture element or in one picture element, and by creating two reorientation states where the major-axis directions of the two states are nearly perpendicular to each other in displaying a white image or a half-tone image, color tones of the two states are offset each other and the viewing angle dependency of color tone change can greatly be reduced.
Moreover, also for gradation inversion, the characteristics of both the minor-axis direction of liquid crystal molecules to be hardly gradation-reversed and the major-axis direction of them to be easily gradation-reversed are averaged and the no-inversion viewing angle range of a gradation level can be expanded.
Thereby, the homogeneity of gradation and that of color tone are averaged or expanded in every direction and wide viewing angle characteristics close to those of a CRT can be realized.
The foregoing and other objects, advantages, manner of operation and novel features of the present invention will be understood from the following detailed description when read in connection with the accompanying drawings.